1. Field of the Invention
The present invention relates to a color television image processing apparatus and method having a color fading reduction function.
2. Description of the Prior Art
Image pickup apparatuses represented by color television cameras generally keep their contrast ranges at specified values by using white compression circuits. This is because large dynamic ranges between highlight portions and lowlight portions of an image acquired by photoelectric conversion cannot fall within the range of signal levels specified by the NTSC system, PAL system or SECAM system. In addition, color television cameras generally convert the R,G,B signals, each corresponding to one of the three primary colors of light, into a composite video signal by a color encoder. The composite video signal is formed by superimposing the color subcarrier signal on the luminance signal, and is used for transmission between studios or the like.
FIG. 1 shows a conventional color television image processing apparatus.
In FIG. 1, reference numeral 1 designates an image pickup device as an image pickup means. R, G and B signals produced from the image pickup device 1 are fed to amplifiers 71R, 71G and 71B. The amplifiers 71R, 71G and 71B, each having a compression circuit and a gamma correction circuit, amplify and compress the R, G and B signals from the image pickup device 1. The compression circuits in the amplifiers compress the video signals that exceed 100% by a factor of 1/5; and the gamma circuits perform the gamma correction with a gamma value of 0.45 to the outputs of the compression circuits. Compression circuits 72R, 72G and 72B compress the video signals R, G and B from the amplifiers 71R, 71G and 71B, respectively. The compression circuits 72R, 72G and 72B compress the output signals that exceed 95% of the output level of the amplifiers 71R, 71G and 71B by a factor of 1/2. Maximum value limiting circuits 73R, 73G and 73B clip the video signals R, G and B from the compression circuits 72R, 72G and 72B at the 110% level of the outputs of the compression circuits 72R, 72G and 72B, respectively. A luminance signal matrix circuit 74 generates a luminance signal from the R, G and B video signals, the maximum values of which are limited by the maximum value limiting circuits 73R, 73G and 73B. The mixing ratios of the R, G and B video signals for generating luminance signal Y are defined by the following equation in the NTSC system: Y=0.30R+0.59G+0.11B. An I color difference signal matrix circuit 75 generates an I color difference signal from the R, G, and B video signals, the maximum values of which are limited by the maximum value limiting circuits 73R, 73G and 73B. The mixing ratios of the R, G and B video signals for generating the color difference signal I are defined by the following equation in the NTSC system: I=0.60R-0.28G-0.32B. A Q color difference signal matrix circuit 76 generates a Q color difference signal from the R, G, and B video signals, the maximum values of which are limited by the maximum value limiting circuits 73R, 73G and 73B. The mixing ratios of the R, G and B video signals for generating the color difference signal Q are defined by the following equation in the NTSC system: Q=0.21R- 0.52 G+0.31 B. A modulator 77 modulates the color difference signal I from the I color difference signal matrix circuit 75 and the color difference signal Q from the Q color difference signal matrix circuit 76 onto a subcarrier. A mixing amplifier 78 mixes the luminance signal from the luminance signal matrix circuit 74 and the color difference signal from the modulator 77, thereby generating a color video signal. The blocks 74-78 constitute a color encoder which operates on the output signals of the maximum value limiting circuits 73R, 73G and 73B undergo so that the color video signal outputs are formed.
The relationship between the signals Y, I, Q and R, G, B is expressed by the following matrix equation (1) when, for example, the compression starting point is specified at 95%, the compression ratio is specified at 1/2, and the maximum value of the input signals to the apparatus is set at 600%. ##EQU1##
FIG. 5A shows an example of input light to the image pickup device 1 using a flesh-colored sample. The input light (1), is a mixture of an R signal of 70% level, a G signal of 50% level, and a B signal of 30% level. The input light (2) is 1.3 times the input light (1), the input light (3) is 1.3 times the input light (2), the input light (4) is 1.3 times the input light (3), . . . , and the input light (9) is 1.3 times the input light (8).
The input light shown in FIG. 5A entering the image pickup device 1 makes the mixing amplifier 78 in FIG. 1 produce the color video signal Vout. The color video signal Vout, when decoded into R, G and B signals, exhibit the cheracteristic curves (1)"-(9)" shown in FIG. 5C.
The conventional color television image processing apparatus described above has a problem in that it causes "color fading" in which an image having less color than a real object appears on the television monitor. The color fading occurs markedly in highlight portions such as flesh-colored portions, that is, high luminance colored portions which are complicated mixtures of red, green and blue that include much luminance signal components and less color difference components. This is because the R, G and B video signals produced from the image pickup device 1 lose much of the color difference components corresponding to R, G and B of the object when the video signals are subjected to the white compression of the highlight portions of the object. This phenomenon is clearly seen by comparing FIGS. 5A and 5C.
In contrast with this, the same object illuminated with less light provides a darker image which does not undergo the white compression. As a result, the image appears on the television monitor with exact colors without color fading.